Condensation heated black body radiation source

ABSTRACT

An isothermal black body radiation source comprises a double reentrant radiating cavity disposed within a heat pipe. Wicking material surrounds the outer surface of the radiating cavity and the heat pipe sidewalls. Heat is supplied to the heat pipe by an electrical winding or other suitable heating means, thereby vaporizing a working fluid. The vapors are condensed on the cavity outer walls thereby releasing their latent heat of vaporization and heating the cavity wall to a uniform temperature. The cavity then radiates energy through an exit aperture. In an alternative embodiment the radiating cavity is disposed within a reflux tube.

United States Patent Stein et al.

'CONDENSATION HEATED BLACK [451 Oct. 17,1972

[ 3,275,829 9/1966 McClune et al. ..250/85 BODY RADIATION SOURCE3,402,761 .9/1968 Swet ..l65/105 X [72] Inventors: Bernard Stein,Andover, Mass;

Sheridan Davis Nashua, NH Przrnary Examzner-James W. Lawrence AssistantExaminer-C. E. Church [73] 'Assignee: Sanders Associates, Inc., Nashua,Attorney Louis Efli 122 Filed: Aug. 19, 1969 I571 ABSTRACT [2| 1 APPL851,203 An isothermal black body radiation source comprises a doublereentrant radiating cavity disposed within a I heat pipe. Wickingmaterial surrounds the outer sur- [52] US. Cl ..250/85 face of theradiating cavity and the heat pipe [51] II!!- Cl. ..H0 5b 11/00sidewalls Heat is supplied to the heat pipe by an elec [58] new Search219/341 165/32 trical winding or other suitable heating means, thereby165/105 vaporizing a working fluid. The vapors are condensed on thecavity outer walls thereby releasing their latent [56] References C'tedheat of vaporization and heating the cavity wall to a UNITED STATESPATENTS uniform temperature. The cavity then radiates energy through anexit aperture. In an alternative embodi- 2,835,48O 5/1958 Pere z..2l9/326 ment the radiating cavity is disposed within a reflux2,952,762 9/1960 Williams et al ..250/85 tube 3,138,697 6/1964 Banca etal. ..250/85 X I 3,229,759 1/1966 Grover ..l65/105 16 Claims, 2 DrawingFigures 28 39 ly l l /l /l l /167% I I 7 /I I /I I /I I /I I O O O Odlififafldiflfliflafimlfi'd'dfi I" m /'/'/,l QQ V i ll l'/I POWER 26 6SUPPLY 7 PATENTEDHCT 11 I972 POWER SUPPLY l la FlGl.

POWER SUPPLY m/vsn/ ro/es B ERNAR D STE IN FIGZ.

' SHERIDAN DAVIS f) @y AGENT CONDENSATION HEATED BLACK BODY RADIATIONSOURCE BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates most generally to the field of radiation sources andin particular to a new and novel black body radiation source.

2. Description of the Prior Art The so-called black body radiationsource has become a valuable laboratory instrument used generally ininfrared technology. Prior to the present invention black body sourcesconsisted of a cone shaped radiator mounted on a thermal mass heated byconduction. One of the most significant of the problems involved withthe use of the prior art black body radiation sources arises fromnon-uniform heating of the radiating cavity which limits the degree towhich the instrument approximates the theoretical black body radiator.The radiation emitted by such sources is of a relatively complexspectrum which does not agree with that theoretically predicted for anisothermal source having a temperature of the average temperature ofprior art black body sources. An additional consequence of non-uniformheating is that the prior art radiation sources do not emit radiationsuch that the exit aperture is a Lambertian radiator. The use of aLambertian diffusing screen is thus required to produce output radiationwhich is a predictable function of angle from the axis of the radiatingcavity. Although black body cavities having several heater windings havebeen developed in an attempt to reduce thermal gradients an exactequalization of temperature has not been achieved in the prior art.

OBJECTS AND SUMMARY OF THE INVENTION It is therefore a primary'objectiveof the present in: vention to provide a new and novel black bodyradiation source.

It is another object of the present invention to provide apparatus ofthe above described character wherein the radiation emitting wallsthereof are maintained in a substantially isothermal condition.

It is an additional object of the present invention to provide apparatusof the above described character wherein the radiation emitting wallsare heated by condensing vapors of a working fluid.

It is also an object of the present invention to provide apparatus ofthe above-described character which radiates energy which is apredictable function of angle from the exit aperture plane.

It is a further object of the present invention to provide an improvedblack body source for operation over temperature ranges of 40 to inexcess of 2,000 C.

It is still a further object of the present invention to provide a newand novel black body cavity using heat pipe principles.

It is yet another object of the present invention to provide a new andnovel black body cavity using a reflux tube heater.

The foregoing, as well as other objectives of the present invention, areaccomplished by providing a radiation cavity defined by a thin heatconducting wall integrally formed as the condenser section of anevacuated evaporative heat transfer chamber. Heat is applied to theevaporator section of the chamber and serves to vaporize a small amountof working fluid within the chamber. The increase in volume due to thephase change creates a minute increase in pressure in the evaporator.This slight pressure produces a flow of the vapors toward the condensersection which is of a lower pressure. In the condenser the vaporscontact the cooler reverse or outside walls of the black body cavity,condense, and give up their heat of vaporization to the cavity. Inlosing heat the vapor volume decreases with an attendant reduction inpressure. Thus, an increased pressure gradient between the condenser andevaporator exists which enhances the vapor flow. The latent heat ofvaporization operates to heat the black body cavity to a very uniformtemperature thus yielding a radiation output which is in very closeagreement with that predicted by radiation theory.

In one embodiment of the present invention a radiating cavity isdisposed within a heat pipe. In an alternative form the cavity isdisposed within a reflux tube evaporative heat transfer chamber.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic cross sectionview of an embodiment of the present invention using a heat pipe.

FIG. 2 is a schematic cross section view of an alternative embodiment ofthe present invention using a reflux tube evaporative heat transferchamber.

DESCRIPTION OF PREFERRED EMBODIMENT Referring now to FIG. 1, there isschematically illustrated in cross section a condensation heated blackbody radiation source fabricated in accordance with the principles ofthe present invention. A radiation cavity 10, such as a double reentrantcone, is formed in one end of a heat pipe 12 and the cone and chambersidewalls are covered with a suitable wicking material 14. The heat pipe12 is degassed, then charged with a working fluid (not shown) andsealed. A heating coil 16 is disposed about the evaporator end 18 of theheat pipe 12 and is coupled to a variable power supply 17. A layer 20 ofinsulation such as asbestos is disposed about the exterior walls of theheat pipe 12, as well as the forward end of the outer case 28, tothereby minimize stray radiation. The black body assembly is thensuspended by a series of clamps 24 and brackets 26 in an outer case 28which may be cooled by an exhaust fan 30, also coupled to the powersupply 17.

In operation, heat is applied to the evaporator end 18 of the heat pipe12 via the heating coil 16. The heat added to the working fluid in theportion of the wick 14 near the coil 16 raises the vapor pressure of theliquid metal working fluid and evaporation results. The increase involume due to the change in phase of the working fluid creates a minuteincrease in pressure in the evaporator end 18 of the heat pipe 12. Thevery slightly pressurized vapor 32 in the evaporator flows towardregions of lower pressure. The condenser is formed by the outside wallof the radiation cavity 10. As the vapors contact the cooler walls ofthe radiation cavity 10 they condense giving up their latent heat ofvaporization. During this change of phase of the working fluid fromvapor back to liquid the volume is decreased and a very slight reductionin pressure occurs and vapor flow from the evaporator to the condenseris enhanced. The condensate then returns by capillary pressure throughthe wick 14 to the evaporator 18, to thereby complete the evaporativeheat transfer cycle. The heat from the condensing vapors is transferredthrough the wick 14 and heats the thin walled radiation cavity 10,resulting in the emission of black body radiation 34 from the exitaperture 36 of the cavity 10. murder to maximize the emissivity of thecavity 10, the radiating walls may be coated with a layer 37 of hightemperature, high emissivity paint or oxide. A low emissivity apertureplate 35 of any desired aperture dimension may be placed about thecavity aperture and preferably is spaced from the insulating layer suchthat'air may circulate there-between.

An accurate temperature sensing means is a necessary element of accuratetemperature control. Since the heat pipe is essentially isothermal, atemperature sensor 38, place in a well 40 within the heat pipe and nearor on the condensing surface of the radiating cavity 10,

allows a very accurate temperature measurementto be maintained. Inpractice, a resistance thermometer or a thermocouple may beused as thesensor and may be coupled to an external temperature indicator 39 orused to complete a temperature control feedback loop.

It is an inherent characteristic of the heat pipe to maintain isothermalconditions within itself. Thus, should a relatively cool spot begin todevelop on the surface of the radiation cavity 10, more vapor tends tocondense at that spot and the isothermal condition is maintained. Theradiation output of the abovedescribed black body source is thus afunction only of temperature and the emissivity of the cavity wallmaterial;.i.e. a single color temperature, and is not made undulycomplicated by local hot and cool spots on the cavity walls as was thecase with prior art.

The Applicants have found that a double reentrant 28 cone radiationcavity 10 to be preferable in the practice of the present inventionsince the 'ratio of radiating wall area to aperture area is optimizedasare internal reflection and overall size. It will beapparent, however,that other cavity configurations, such as a cylinder, cone, sphere, orothers of more complex geometry are equally. applicable in the practiceof the present invention. The cavity walls should be as thin aspracticalin order to minimize temperature gradients between the outercondensing surface and the inner emitting surface.

The apparatus of FIG. 1 functions safely at pressures between 0.01 and2.0 atmospheres, however, an operatingpressure of 1.0 atmosphere ispreferred in order to eliminate the requirementof a pressure vesselchamber. The-temperature range of operation is a function of the workingfluid properties and of the pressure limitations of the heat pipematerials. The useful life of the apparatus of the present invention isalso dependent upon the materials of which it is fabricated. Atthehigher operating temperatures materials which would normally becompatible with one another may attack one anotherand spontaneousoxidation occurs with mostmaterials in contact with the atmosphere attemperatures in excess of l,l00 C. 0f the available liquid metal workingfluids'lithium and sodium show the most promise in having relativelyhigh heats of vaporization and relatively wide temperature range as aliquid, however, cesium, bismuth, lead, lead-bismuth, potassium, tin andsodium-potassium may also be useful in the practice of the presentinvention. For use in a very high temperature; i.e. about 2,000 C.,embodiment of the present invention, silver has high potential as aworking fluid. These elements, however, are in general very chemicallyactive and at the elevated temperatures in-. volved such materials asstainless steels, niobium-zirconium alloy, tantalum and molybdenumare'the primary containment materials. In one embodiment of theinvention actually fabricated by the Applicants, the

' heat pipe 12 and radiating cavity 10 were fabricated of AISI304Lstainless steel. The wick 14 was formed of six layers of 160 mesh316 stainless steel welded to the outer surface of the radiating cavity.This-wick material is wetted by a sodium working fluid but has a highresistance to its deleterious effects, It will be apparent that the heatpipe must. have some internal capillary structure. In this specificembodiment a'stainless steel screen was employed, however, other porousforms of wicking material are equally applicable to the practice of the.present invention. An alternative approach would involve the use of afine grooved capillary structure integrally formed in the chamber andcavity walls. The heating coils 16 around the evaporator section may beof any suitable type. Induction coils offer the advantage of operatingat temperatures below the evaporator temperature, however, the requiredpower supplies are generally more expensive than resistance heaters.

The same heat pipe principles as discussed above may also be applied toa low temperature radiation standard should such a device be desired.The electrical heating coilsof FIG. 1 are merely replaced with a coolingcoil coupled to a recirculating cooler filled with a cryogenic fluidsuch as liquid helium. A working fluid such as liquid nitrogen is placedin the heat transfer chamber and is cycled through the apparatus in amanner which is simply reversed from that described with reference toFIG. 1. The working fluid is evaporated from the outside surface of thecavity, condensed at the cooling coils and returns to the cavity via thewick by capillary pressure. In this manner the cavity walls aremaintained at an isothermal cryogenic temperature.

Turning .to FIG. 2, there is illustrated in schematic cross section analternative embodiment of a condensation heated black body radiationsource wherein a reflux tube is used as an evaporative heat transferchamber. In this embodiment a radiation cavity 210 is disposed in theupper end of a reflux tube 212. The tube and the front face of theprotective case 228 are insulated with a layer 220 of asbestos or othersuitablev insulating material. The tube is mounted in case 228 on alayer of insulation 229 and is secured by means. ofa series of brackets226 and clamps 224. A low emissivity aperture plate 235 of any desiredaperture dimensions may be placed about the exit aperture of theradiation cavity 210. This plate is cooled by convection andsubstantially eliminates any secondary radiation from the protectivecase 228. A pool of liquid metal working fluid 215 in the bottom ofevaporator section 218 of the tube 212 is heated such as by a coil 216thereby raising heat of vaporization to the cavity walls. This energy isthen reradiated as radiation 234 by the cavity walls through the exitaperture 236. Radiation baffles 237 and 239 are placed in the refluxtube 212 to prevent the inside surfaces of the cavity 210 from absorbingdirect radiation from the evaporator section 218 and to prevent anysputtered liquid from directly contacting the cavity walls. An exhaustfan 230 may be disposed in the rear of the protective case 228 topreclude unwanted heat build up within the case and may be driven by thesame powersupply 217 as is used to operate the heating coils 216. Atemperature sensor of the same type and installation as shown in FIG. 1may be used in this embodiment of the invention but is not shown in FIG.2 for the purpose of clarity.

In both illustrative embodiments of the invention shown in the appendeddrawings, the temperature uniformity over the radiation cavity walls isvery nearly isothermal. This is due to the. fact that the heat flowsfrom regions of higher temperature to regions of lower temperature andthe heat conduction occurs at a rate which is proportional to thetemperature difference. Since large quantities of heat may betransferred over very small temperature differences any thermalgradients which may start to develop on the surface of the radiationcavity will automatically be compensated by an increased heat flow tothe relatively cooler area.

' In the same manner any hot spots which begin to develop willautomatically receive a lesser quantity of heat and'the substantiallyisothermal surface is maintained. The isothermal nature of the radiatingsurface, thus, emits a relatively clean spectrum which fits very closelywith that predicted by radiation theory. In addition, since each elementof the surface is at the same temperature, the exit aperture closelyapproximates a true Lambertian radiator and the resulting radiation isalso predictable as to its spatial characteristics.

It will thus be seen that the objectives set forth above, among thosemade apparent from the preceding description are efficiently attainedand since certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatter contained in the foregoing description or shown in theaccompanying drawings shall be interpreted as illustrative and not inalimiting sense.

Having described what is new and novel and desired to secure by LettersPatent, what is claimed is:

1. An improved black body radiation source comprising an evaporativeheat transfer chamber having an evaporator portion and a condenserportion,

a working fluid disposed within said chamber,

means for heating the evaporator portion of said chamber to therebyvaporize a portion of said working fluid,

a radiation cavity having an exit aperture, disposed in the condenserportion of said chamber, and having an outer surface whereon vaporscondense thereby heating said cavity to a substantially uniformpreselected temperature such that radiation is emitted through saidaperture, and

means for returning the condensate to said evaporator portion of saidchamber.

2. Apparatus as recited in claim 1 wherein said evaporative heattransfer chamber is a heat pipe having a capillary structure disposedabout the outer surface of said cavity and the sidewalls of said chamberwhereby said condensate is returned from said condenser portion to saidevaporator portion by capillary pressure.

3. Apparatus as recited in claim 2 wherein said capillary structure is awick formed of a material which is wetted by but nonreactive with saidworking fluid.

4. Apparatus as recited in claim 3 wherein said wick is formed of aplurality of layers of stainless steel screen.

5. Apparatus as recited in claim 1 wherein said evaporative heattransfer chamber is a reflux tube having a condenser portion which iselevated with respect to said evaporator portion whereby said condensateis returned from said condenser portion to said evaporator portion bygravity.

6. Apparatus as recited in claim 5 further including means disposedwithin said reflux tube between said evaporator and said condenser forpreventing direct contact of said working fluid in the liquid phase withsaid radiation cavity.

7. Apparatus as recited in claim 1 wherein said working fluid is aliquid metal.

8. Apparatus as recited in claim 7 wherein said liquid metal is selectedfrom the group of liquid metals consisting of: lithium, cesium, sodium,bismuth, lead, lead-bismuth potassium, tin, and sodium-potassium. 9.Apparatus as recited in claim 1 wherein said working fluid is silver.10. Apparatus as recited in claim 1 further including a layer of highemissivity material disposed on the inner surface of said radiationcavity. 11. Apparatus as recited in claim 1 further including a lowemissivity aperture plate disposed about said exit aperture of saidradiation cavity. 12. Apparatus as recited in claim 1 further includingtemperature sensing means disposed proximate the outer surface of saidradiation cavity. 13. Apparatus as recited in claim 12 wherein saidtemperature sensing means is coupled to and operative to control theoutput of said heating means. 14. Apparatus as recited in claim 1further including means for insulating said condenser portion of saidchamber except for said radiation cavity exit aperture to therebysubstantially preclude the emission of radiation from any portion ofthe'apparatus except said aperture. 15. Apparatus as recited in claim 1wherein said radiation cavity is a double reentrant cone cavity. 16.Apparatus as recited in claim 15 wherein said double reentrant cone hasa divergence of 28.

1. An improved black body radiation source comprising an evaporativeheat transfer chamber having an evaporator portion and a condenserportion, a working fluid disposed within said chamber, means for heatingthe evaporator portion of said chamber to thereby vaporize a portion ofsaid working fluid, a radiation cavity having an exit aperture, disposedin the condenser portion of said chamber, and having an outer surfacewhereon vapors condense thereby heating said cavity to a substantiallyuniform preselected temperature such that radiation is emitted throughsaid aperture, and means for returning the condensate to said evaporatorportion of said chamber.
 2. Apparatus as recited in claim 1 wherein saidevaporative heat transfer chamber is a heat pipe having a capillarystructure disposed about the outer surface of said cavity and thesidewalls of said chamber whereby said condensate is returned from saidcondenser portion to said evaporator portion by capillary pressure. 3.Apparatus as recited in claim 2 wherein said capillary structure is awick formed of a material which is wetted by but nonreactive with saidworking fluid.
 4. Apparatus as recited in claim 3 wherein said wick isformed of a plurality of layers of stainless steel screen.
 5. Apparatusas recited in claim 1 wherein said evaporative heat transfer chamber isa reflux tube having a condenser portion which is elevated with respectto said evaporator portion whereby said condensate is returned from saidcondenser portion to said evaporator portion by gravity.
 6. Apparatus asrecited in claim 5 further including means disposed within said refluxtube between said evaporator and said condenser for preventing directcontact of said working fluid in the liquid phase with said radiationcavity.
 7. Apparatus as recited in claim 1 wherein said working fluid isa liquid metal.
 8. Apparatus as recited in claim 7 wherein said liquidmetal is selected from the group of liquid metals consisting of:lithium, cesium, sodium, bismuth, lead, lead-bismuth potassium, tin, andsodium-potassium.
 9. Apparatus as recited in claim 1 wherein saidworking fluid is silver.
 10. Apparatus as recited in claim 1 furtherincluding a layer of high emissivity material disposed on the innersurface of said radiation cavity.
 11. Apparatus as recited in claim 1further including a low emissivity aperture plate disposed about saidexit aperture of said radiation cavity.
 12. Apparatus as recited inclaim 1 further including temperature sensing means disposed proximatethe outer surface of said radiation cavity.
 13. Apparatus as recited inclaim 12 wherein said temperature sensing means is coupLed to andoperative to control the output of said heating means.
 14. Apparatus asrecited in claim 1 further including means for insulating said condenserportion of said chamber except for said radiation cavity exit apertureto thereby substantially preclude the emission of radiation from anyportion of the apparatus except said aperture.
 15. Apparatus as recitedin claim 1 wherein said radiation cavity is a double reentrant conecavity.
 16. Apparatus as recited in claim 15 wherein said doublereentrant cone has a divergence of 28*.